The invention relates to a hydrodynamic bearing.
FIG. 3 is a longitudinal section view showing an example of a hydrodynamic bearing of the prior art. The hydrodynamic bearing of the prior art comprises: a shaft 31 which erects from a bed 30 and on which a circumferantial flange 33 is radially formed; a sleeve 32 which is disposed so as to surround the shaft 31 and upper and lower faces 33a and 33b of the flange 33; a radial dynamic pressure bearing portion 35 which is formed between the shaft 31 and the sleeve 32, by dynamic pressure grooves 36 formed in the outer surface of the shaft 31; and first and second axial dynamic pressure bearing portions 37 and 38 which are formed between the upper and lower faces 33a and 33b of the flange 33 and the sleeve 32, by dynamic pressure grooves 39 and 40 formed in the upper and lower faces 33a and 33b of the flange. The sleeve 32 is vertically divided into two portions, i.e., first and second sleeves 32a and 32b which pressingly hold the flange 33 in a vertical direction. The first axial dynamic pressure bearing portion 37 is formed between the first sleeve 32a and the flange 33, and the second axial dynamic pressure bearing portion 38 is formed between the second sleeve 32b and the flange 33. Conical faces 41 and 42 having a diameter which is gradually reduced as moving toward the atmosphere side are formed on the end portions of the shaft 31 on the atmosphere side, respectively. The spaces between inner cylindrical faces 43 and 44 of the first and second sleeves 32a and 32b which are opposed to the conical faces 41 and 42 serve as accommodating portions 45 and 46 which accommodate a liquid in the bearing, respectively.
In the hydrodynamic bearing of the prior art, when the shaft 31 relatively rotates with respect to the sleeve 32, the liquid L in the bearing expands as the temperature rises, to be accommodated in the liquid accommodating portions 45 and 46, thereby preventing the liquid from leaking to the outside. Since the inner side face 43 of the first sleeve 32a is a cylindrical face, the inclination angle .beta. of the conical face 41 of the shaft 31 cannot be set to be large, as shown in FIG. 4. This is because, when the inclination angle .beta. is made large, the space between the conical face 41 of the shaft 31 and the cylindrical inner side face 43 of the first sleeve 32a is made excessively large, thereby causing the liquid L to leak, Therefore, the accommodation amount of the liquid accommodating portion 45 is reduced. Furthermore, the diameter of the liquid accommodating portion 45 is gradually increased as moving toward the atmosphere side, and hence the liquid is susceptible to the centrifugal force, These produce a problem in the leakage of the liquid inevitably easily occurs. Although the problem of the upper accommodating portion 45 of the shaft 31 has been described with reference to FIG. 4, also the lower accommodating portion 46 of the shaft 31 has the same problem.